Solvent based cements have long been used to bond together thermoplastic pipe fittings. The solvent based cement may be broadly divided into at least two categories: the solvent, and the resin or dopant. The solvent acts to partially solvate and swell the pipe or other substrate to be bonded. After a period of time, the cement cures by evaporation of solvents from the pipe or other substrate into the atmosphere. The resin dissolved in the adhesive dries upon cure into the surface irregularities caused by the solvent, thus providing a strong, tight, leakproof joint.
A typical solvent based cement comprises a thermoplastic resin, usually the same type of resin as the substrates to be adhered, such as PVC, CPVC, and ABS, and a small group of solvents approved by the National Sanitation Foundation (NSF) for use in potable water pipe systems which transport water for drinking; drain, waste and vent (DWV) in homes and industry; turf and sprinkler systems; and jacuzzis, spas, and tubs. The group of approved solvents are:
a) methyl ethyl ketone; PA1 b) tetrahydrofuran; PA1 c) dimethyl formamide; PA1 d) acetone; PA1 e) cyclohexanone; and, PA1 e) N-methyl-2-pyrrolidone.
Efforts have been underway to reduce the amount of solvents evaporated into the atmosphere to control air pollution, reduce fire hazards, and reduce potential occupational hazards. For example, the South Coast Air Quality Management Division (SDAQMD) has issued Rule 1168 which provides limits to volatile organic compound (VOC) emissions. In PVC or CPVC based cements, the VOC emissions were to have been reduced from the then existing 850 g/l level to 450 g/l by Jan. 1, 1994, and further to 250 g/l by Jan. 1, 1198.
Early efforts to reduce VOC emissions from cements focused on merely increasing the levels of thermoplastic resin, such as PVC or ABS, while simultaneously decreasing the amount of solvent. The maximum concentration of PVC in cements using the NSF approved solvents has been at an upper limit of around 14 weight percent because greater amounts of resin dissolve only with great difficulty. Further, in order to comply with ASTM 2564-88, PVC solvent cements must be capable of dissolving at least 3 weight percent of additional PVC at 23.degree. C. without signs of gelation. At amounts greater than 14 weight percent, PVC usually begins to show signs of gelation when 3 weight percent more PVC is added in typical solvent formulations.
U.S. Pat. No. 5,252,634 describes hollow, silica-aluminum alloys known as Zeelon Z-Light spheres, mixed with PVC, CPVC, or ABS solvents cements to increase the solids content while reducing the VOC volume. It would be desirable, however, to formulate a solvent based cement without having to use additives which would increase the cost of the cement. U.S. Pat. No. 4,675,354 proposes adding N-methyl-2-pyrrolidone as essentially the sole solvent in a PVC, ABS, polyacrylate, polycarbonate, cellulose acetate, polyacrylamide, polyamide, or polystyrene based cements to reduce VOC emissions, flammability, and occupational hazards. U.S. Pat. No. 4,687,798 proposes a combination of a lactam such as N-methyl-2-pyrrolidone with an alkanol or an ester of an alkanoic acid as a solvent system using the aforementioned resins as the dopant. This patent reports that the cement using the solvent combination meets or exceeds ASTM lap shear strength requirements. Also described in EP 140188 is a removable coating on the surface of vehicle tires where the coating is made of a thermoplastic polyurethane elastomer dissolved in, for example, 70 to 90 weight percent of N-methyl-2-pyrrolidone. We have found, however, that solvent cements having the described large quantities of N-methyl-2-pyrrolidone fail to satisfy the hydrostatic burst strength requirements Under ASTM D-2564.